Agricultural combines are typically equipped with large front drive wheels that support the bulk of the weight of the agricultural combine as it travels through the field harvesting crops. They're also typically equipped with rear steerable wheels that are smaller in diameter than the front wheels and are configured to carry a smaller load.
One of the problems faced by agricultural combines is the need to maintain good contact with the ground as the terrain changes underneath the wheels, and also to minimize ground compaction as much as possible. If good contact is not maintained, wheels can spin and the agricultural combine can stall in the field. In addition, if the load shifts from wheel to wield added force on a wheel can call as the ground under that wheel to become unnecessarily compacted.
A traditional way to alleviate these problems has been to mount the rear steerable wheels on a pivoting axle. The axle is typically arranged to pivot about a pivotal axis of the agricultural combine that extends longitudinally and horizontally, parallel with the direction of forward travel as the agricultural combine is driven through the field. If the terrain over which the two large front drive wheels passes causes the agricultural combine to roll to the left, for example, the combine body will pivot about the longitudinal pivot axis of the rear axle. In this manner, the load is evenly distributed over the 4 wheels as much as possible and all four wheels maintain good contact with the ground.
The ability of the rear axle to pivot is not unlimited, however. Typically there are mechanical stops mounted to the axle, or to the frame above the axle at each end of the axle adjacent to the rear wheels of the agricultural combine. Without the stops, the combine could travel over rough terrain sufficient to cause the axle to pivot until one of the rear wheels rubs against the side of the combine and its mechanical components. This can cause the mechanical components to fail.
No one axle oscillation stop is suited for every situation, because wheels having tires of a variety of widths and heights can be used on a combine. If the dimensions of the wheel and tire change, the position of the axle oscillation stop should change as well. By adjusting the stop when the wheels and tires are changed, the maximum range of movement of the axle can be provided while still preventing the wheel and tire from damaging the side of the combine.
Furthermore, axles of agricultural combines are typically made so they can be extended or retracted. If an axle is extended (for example to permit a larger wheel and tire to be used) it is desirable to adjust the axle oscillation stops as well.
Unfortunately, there is no easy way to determine the best axle oscillation stop position when changing wheels, tires, and adjusting the length of the axle. As a further complication, future combines are now being proposed that we'll have not only a pivotable rear axle, but an automatically extendable and retractable rear axle. In these future combines, the rear axle of a combine can be extended and retracted, as necessary, to provide even where, or to provide steering clearance between the rear wheels and the sides of the combine automatically as the wheels are steered.